The present invention relates generally to ink jet printing and more particularly to ink jet color printing.
A typical ink jet color printer has a print head which is movable back and forth relative to a recording medium, e.g. a sheet of paper, in a main scanning direction. A plurality of nozzle arrays, at least one for each color, are mounted on the print head side-by-side in the main scanning direction. Each nozzle array has a plurality of nozzles arranged in one or more rows which extend in a sub-scanning direction in which the recording sheet is fed past the print head, i.e. a direction orthogonal to the main scanning direction. In order to print an image on the recording sheet, ink droplets are expelled from the various nozzles, so that dots (pixels) are formed on the recording sheet. The positions of the dots formed on the recording sheet depend on the mechanical structure of the print head. Further, the position in the main scanning direction depends on the timings at which the nozzles are energized during the continuous movement of the print head, whereas the positions in the sub-scanning direction depend on the feed distance over which the recording sheet is fed after each scan pass of the print head.
In order to obtain an artifact-free printed image of high quality, it is necessary that the dots are formed on the recording sheet with high positional accuracy. This is particularly the case in a color printer, because colored seams would be visible in the printed image if the positions of the dots of different colors, which are formed by different nozzle arrays, were not adjusted correctly. In addition, even in a mono-color printer, positional deviations of the dots in the sub-scanning direction would result in the occurrence thin lines with reduced or increased image density which separate the image areas that are formed during subsequent scan passes of the print head.
In a so-called bubble-jet printer, the ink droplets are formed by heating the liquid ink, so that part of the ink is evaporated abruptly and creates a pressure which causes an ink droplet to be expelled from the nozzle. In a so-called hot melt ink jet printer, the ink is solid at room temperature and has to be heated above its melting point when the printer is operating. In this type of printer, the pressure for expelling the ink droplets is typically created by means of piezoelectric actuators. In any case, the print head will be subject to temperature changes, and these temperature changes will influence the operating conditions of the print head.
U.S. Pat. No. 5,864,349 discloses an ink jet printer in which a temperature sensor is mounted on the print head for monitoring the operating conditions of the print head. U.S. Pat. No. 4,544,931 and U.S. Pat. No. 5,477,245 disclose ink jet printers in which the signal of a temperature sensor mounted on the print head is used for controlling the frequency or pulse width of pulses with which the nozzles of the print head are energized. JP-A-60 222 258 discloses an ink jet printer in which a print skew detector is mounted outside of the margin of the recording sheet, and the print head is controlled to print dots on this detector during both the forward and the return scan pass of the print head. By comparing the positions of the dots formed in the forward and return scan passes, the detector monitors the effect of a skew of the ink droplets which is caused by the movement of the print head. When, due to temperature and moisture, any change in the conditions of the nozzles and the print head carriage leads to a positional deviation of the dots formed in the forward and return strokes of the print head, the detector will indicate these deviations and will cause the control system of the printer to perform an appropriate correction.
It is an object of the present invention to reduce the influence of the temperature of the print head on the positional accuracy of dot formation without any need for complex detection systems. According to the present invention, this object is achieved by a method of controlling the ink jet printer.
The invention is based on the consideration that the influence of the temperature of the print head on the positions where the dots are formed on the recording medium is mainly due to thermal expansion of the print head. According to the general concept of the invention, at least one temperature sensor on the print head is used for monitoring the temperature of the print head or the temperature distribution within the print head, so as to predict the effect of thermal expansion of the print head on the nozzle positions on the basis of the known thermal expansion behavior of the print head. Then, the predicted thermally induced positional offsets of the nozzles are compensated for by an appropriate control of the printer. Thus, it is sufficient to provide one or more temperature sensors for making the printer more robust against temperature changes of the print head and for improving the positional accuracy in the dot formation.
In general, the print head will undergo thermal expansion in all three dimensions and, as a result, the positions of the nozzles may be offset in the sub-scanning direction (X-direction), the main scanning direction (Y-direction) and also in a direction normal to the plane of the recording medium (Z-direction). Even an offset in the Z-direction may influence the positions of the dots, because it influences the distance between the nozzle and the recording medium and hence the time of flight of the ink droplets. Since, due to the movement of the print head, the ink droplets have a velocity component in the main scanning direction (skew), an offset in the nozzle position in the Z-direction will lead to an offset in the dot position in the Y-direction. As the print head moves in the Y-direction, the deviations of the dot position in this Y-direction caused by nozzle offsets in the Y- and Z-directions can be compensated for by appropriately correcting the timings at which the nozzles are energized.
Offsets of the nozzle positions in the X-direction can be compensated for by appropriately correcting the feed distance of the recording medium. More specifically, when a nozzle array has a row of nozzles extending in the X-direction, the feed distance of the recording sheet between two subsequent scan passes of the print head must be equal to the distance between the first and the last nozzle of the row plus the distance between two immediately adjacent nozzles of the row. Since these distances, especially the comparatively large distance between the first and the last nozzle, may vary in response to temperature changes, the feed distance of the recording sheet should be adapted accordingly.
In addition, depending on the structure of the print head, thermal expansion of the mounting structure of the print head may also cause a shift of the nozzle array, as a whole, in the X-direction. As long as the temperature is essentially constant over the time which is needed for printing one page, this shift will only lead to a minor shift of the printed image as a whole on the recording sheet and may be neglected. If, however, substantial temperature changes occur between two printing operations in immediately adjacent or overlapping image areas, then this total shift of the nozzle array should be compensated for as well.
It will generally depend upon the structure of the print head and its mounting structure and on the required level of accuracy as to whether the nozzle offsets in all three directions, X, Y and Z or only selected ones of these offsets need to be compensated for.
The term xe2x80x9ctemperature sensorxe2x80x9d, as used in the description given above, should be interpreted in a broad sense. More precisely, what actually needs to be measured is a parameter that is correlated to the thermal expansion of the print head and thus permits a determination of the thermally induced offsets of the nozzle positions. In many known temperature sensors, the principle of temperature measurement is itself based on the measurement of the thermal expansion of a medium whose thermal expansion coefficient is known. Thus, it is also possible, according to the present invention, to measure the temperature-dependent distance between two predetermined points on the print head and to take this distance as a parameter which implicitly indicates the temperature of the print head and thereby permits a determination of the thermally induced positional offsets of the various nozzles.
In a preferred embodiment, a predetermined point on the print head is taken as a reference position in the Y-direction, and the absolute position of this point is directly measured with a linear encoder. Then, the Y-positions of the various nozzles are given as temperature-dependent distances between the nozzles and the reference position.
To determine the positions of the nozzles in X- and Z-directions, the print head may be mounted slidably on a guide rail which defines a fixed reference position in the X- and Z-directions, so that the X- and Z-coordinates of the nozzles can again be given by temperature-dependent distances to the respective reference positions.
If the temperature of the print head as a whole can be assumed to be uniform and if the structure of the print head which determines the thermal expansion behavior is made of only a single material, e.g. aluminum, the temperature may be measured with a single temperature sensor, and the temperature-dependent relative positions of the nozzles may be calculated from the known thermal expansion coefficient of this material. On the other hand, if the print head is composed of different materials, then the different thermal expansion coefficients of these materials may be taken into account in the calculation. As an alternative, it is possible to measure the relative positions of the nozzles at different temperatures in advance and to store the results in a look-up table in the control system of the printer.
If it is expected that the temperature of the print head will, in operation, be non-uniform, then it is possible to employ a plurality of the temperature sensors, so that the temperature distribution within the print head can be determined with sufficient accuracy by interpolation techniques, and the local thermal expansions can be calculated on the basis of this temperature distribution.